The present invention relates to improvements in a position detecting apparatus for detecting a displacement amount between relatively displacing two parts such as a scale and a sensor applied to a machine tool or precision measuring apparatus, and more particularly to an position detecting apparatus which is simplified in structure and improves a control accuracy of a servo motor thereof.
Generally, a position detecting apparatus for a machine tool or precision measuring apparatus employs a servo mechanism in order to correspond a relative position of two relatively displacing parts with a target position. An alternating servomotor is mainly applied to such a servo mechanism. In order to control an alternating servomotor, it is necessary to supply digital two-phase signals for controlling position and speed and digital three-phase signal for controlling magnetic field thereof. The two-phase signals for controlling position and speed should be a relatively short cycle signal having a micrometer unit length in order to enable a precise control. The three phase signals for controlling the magnetic field should be a relatively long cycle signal having a cycle corresponding to the pitch of slots of the servomotor. The pitch of the slots is generally several millimeters, and therefore the cycle of the two-phase signals is largely different from that of the three phase signals. Therefore, conventional position detecting apparatuses are generally arranged to have a first scale having graduations corresponding to the cycle of the two-phase signals for position and speed control and a second scale having graduations corresponding to the cycle of the three-phase signals for the magnetic field control. Further, in case that a linear alternating servomotor is employed in a position detecting apparatus, it is necessary to provide two encoders for the two phase signals and the three phase signals.
However, such provision of two scales and two encoders generates problems such that a structure of the serve mechanism is complicated and that adjusting operations for installation thereof and assembling operations thereof are increased. Further, it is difficult for the position detecting apparatus having such construction to accurately correspond the phase difference of the two-phase signals and the three-phase signals.